Current transformer



Jan. 5, 1960 G. w. sPlcER 2,920,297

l CURRENT TRANsFoRMER Filed April 28, 1955 Trae/V575 United StatesPatent O A 2,920,291 CURRENT TRANsFoRMER George-W. Spicer, Philadelphia,Pa., assignor to I-T-E ACircuit Breaker Company, Philadelphia, Pa., acorpo- Y ration of Pennsylvania Application April 2s, 1955, serial No.504,454 z claims. (ci. 336-174) secondary. In order that the secondaryremain properly and securely positioned with respect to the primary, ithas been necessary to provide metallic feet or other external supportsfor the secondary windings. This arrangement, although satisfactory formany installations, has been found to be a rather cumbersome one,particularly when the space available for installation of the`transformer is rather limited. Moreover, under crowded orcrarnpedconditions, the relaying accuracy of such transformers, when constructedwith separate coils instead of a toroidal winding, was severely limited.

.Y ,I propose to overcome these and other diiiiculties inherent in thedevices of the prior art by providing a current transformer in whichthe'secondary is rigidly secured to the bushing type insulating tube ofthe primary conductor by means of a cast epoxy resin.

Accordingly, i-t is an object of my invention to provide va novelcurrent transformer particularly suitable for use in installations wherethe available spaceis limited. j

. It is another object of my invention to provide a current transformerof high relaying accuracy and having excellent mechanical and dielectricproperties.

A further object of my invention is to provide a current transformer inwhich the primary insulating tube is rigidly secured to the secondary byan epoxy resin thereby resulting in an integral unit suitable forinstallations of limited space.

These and other objects of my invention will become more apparent whentaken in connection with `the following description and drawings inwhich:

Figure 1 is a view showing, in perspective, my novel current transformerwith a portion of the secondary cut away to show the constructionthereof.

Figure 2 is a cross-section taken along the lines 2 2 of Figure 1.

Essentially, my invention involves a current transformer in which thetoroidally wound secondary turns are rigidly secured to the insulatingtube provided for the primary conductor by a body of cast epoxy resin.

By epoxy resin, I refer to the reaction product of epichlorhydrin andBis-phenol A, which forms a resin having superior heat stability,chemical resistance and electrical properties.

These materials are sold under various trade names such as Epon,"Araldite or HysoL Although essentially they are the reaction products ofepichlorhydrin and Bis-phenol A, they may also be the reaction productof any polyphenol and polyhalohydrin. Compounds of the type to which Irefer are described, for example in United States Patent Nos. 2,324,483,2,458,796 and 2,637,- 716.

2,920,297 Patented Jan. 5, 1960 These epoxy resins are readily cast intodesired shapes Vwith a minimum of shrinkage during curing, therebymaking a cast product having a minimum of internal stresses. Moreover,the cast resin readily adheres to other materials, has excellentmoisture and chemical resistance, and high mechanical strength. Forpurposes of economy, and to reduce therrnal'expansion, inorganic llerssuch as slate powder, kaolin, quartz, talcum, silica, mica may be addedto the resin mix. Generally fillers are added in amounts such that theyconstitute between about 60 and 70 weight percent of the nal mixture.

Referring now to the drawings, the primary of the transformer to which Irefer comprises a hollow bushing- Atype sleeve or insulating tube 11which can readily be placed laround a single-turn conductor such as abus bar. The secondary is inthe shape of a doughnut and contains adesired number of turns of a toroidally-wound wire which is wound aroundiron core 14.

The core is constructed of laminated steel as shown in Figure 2. Betweenthe iron core and the wire coil is a layer of tape insulation 15. Afterthe coil 20 is wound about insulation 15 another layer of tapeinsulation 17 is wound around the entire assembly. The tape which isused in insulating the secondary may be conventional vinsulation tapehaving a cotton or polyvinyl chloride base.

An insulation tube 11 assumes a shape so that it Awill t securely to theprimary conductor. The insulating material used here is an electricalgrade phenolic kraft paper of the type commonly used for electricalinsulation. The insulating paper is free from impurities which may leadto heat distortion and is of rather high dielectric strength. Moreover,the paper should not contain iillers which would make it hygroscopic.

. Y The secondary windings 20 are wound in a toroidal fashion so thatthe secondary is of a doughnut shape.

In bonding the primary and secondary the transformer windngsandinsulating bushing are placed in position and then dried in acirculating oven at 250 F. maximum temperature. Y, Theiepoxy resin whichis cast supplied the bond between the insulating tube 11 and secondary20 and may be cured with either acid or amine hardeners.

As previously mentioned, these resins go under trade names such as Epon,Araldite or HysoL The utilization of such resins in a hermeticallysealed resistor is described in United States Patent No. 2,685,016 whichis owned by applicants assignee. This resin has the properties of highadhesion to the solid material of the coil form and the ability to beconverted to a solid at a safe temperature for the material of theseforms. A specific example of the resin which I may employ goes under thetrade name of Hysol Resin No. 6800, manufactured by the HoughtonLaboratories. This resin is of the heat setting type and is melted atabout 300 F. The resin contains a quartz filler in an `amount such thatthe final composition comprises quartz 60 percent by weight and epoxyresin 34 percent by weight.

Au acid hardener such as phthalic anhydride is then added in order toobtain curing. Specifically, the hardener used goes under the trade nameof Araldite Hardener No. H N. 901. The assembly containing 3.7 pounds ofresin has added thereto 0.4 pound of hardener. 'Il-ie amount ofhardener, of course, may vary with the rate of curing desired and willnormally be between about 5 to 15 percent of weight of the resin used.

The assembly is cast by pouring the resin under a minimum vacuum of 28inches of mercury. It is then cured in a circulating oven maintainedbetween 250 and 260 F. for 14 to 18 hours. Upon cooling to roomtemperature the assembly is ready for use.

Referring again to Figure 1 it can be seen that two leads 22 and 23emanate from the secondary coil. These may be of such a length so as toconnect directly with the u'ysual electrical apparatus. They may alsobe` conepoxy resin rigidly secures it to the outer insulated` sur? `face32 of the secondary and to the surface 33 of the phenolic paper and thebushing-type insulating tube -11.

The doughnut shaped secondary 20 is then arranged so that the axis orcenterline of the doughnutvcoincides -with the vaxis of the insulatingtube. The resin and secondary have a disc-like shape and'preferably thefresin isffcast yso that surfaces 40 and 41, which lare also normal -tothe axis of the insulating tube, are smooth and parallel. Thisconnection of the inner circumferential surface Vof the 'secondary tothe primary insulating tube yeliminates the need for metallic feet orother supports forth'e secondary which have been necessary in devicesYof the prior'art.

These metal supports have had a tendency to reduce the f i `insulationlevel in tig/ht andcramped locations'andfthereby limit the relayaccuracy of transformers of "this type.

.The strongly adhesivefepoxy resinis thereforevuse'd as a mechanicalsupport between the insulating tube 11 and {transforrner by supportingthe'transformeron the'insulating tube rather than on any'external'elementsi In addition to the outstanding mechanicalkproperties ofthe epoxy resins their electrical properties also `finduse. Their dielectric properties also are useful iny that the resin islocated in the dielectric e'ld'between the l'primary conductor and theVsecondary winding. Thus, the properties of epoxy resins to set upfree-of-airvoids is also important, since the epoxy forms between thesurface path from the conductor to the grounded secondary.

My transformer, therefore, having anlinsulated sleeve `11 for theprimary and a secondary 20 interconnected by a cast epoxy resin providesla Verycompact type transformer with excellent insulating properties.

In the foregoing, I have described my invention only in connection withpreferred embodiments thereof.

'Many variations and modifications ofthe principles of myinventionwithin the scope ofthe description herein 'are obvious.Accordingly, I prefer to be bound not by the lspecific disclosureherein, ,but onlyby the appending claims.

I claim: Y

l. A current transformer comprising -ahollow insulating tube suitablefor encasing a primary conductor, a toroidally Wound insulated-"secondary surrounding said tube, and a radial connection rigidlysecured to the inner circumferential surface of the secondary and theouter surface of the insulatingtube; said insulating tube comprising anelectrical grade phenolic kraft paper; said secondary comprising a'doughnut-shaped iron core, a first layer of insulation adjacent tofsaidcore, la conductive Winding adjacent tosaid first layerof insulationland a second layer of insulation adjacent -to5said conductive Winding;said radial connection comprisinga cast epoxy resin having ste VT()vpercent of Ianinorganic filler incorporated therein.

2. The transformer as set forth in claim 1in which the epoxy surroundsthe tube and is interposed between the tube and the in nercircumferential surface of the secondary; said v epoxyr resin being castso that ltheFepoXy includes 'two parallelrelatively smooth surfacesnormal -to lthe 4axis 'of 'the ins'ulating tube; fsaid parallel surfacesAappearing as Iextensions of the 'secondaryfouter surface.

` References rCited' in Vtheiile of .this patent f UNITED STATES PATENTSFrance '.Nov. :20, 192

OTHER --REFERENCES Ethoxylines-A New Group of Triple-'Function Resins,Electrical Manufacturing, July' 1949, 4xp'p. '78481.

EpoXyCasn'ng Resins in Electronics? Modern `Plastics, July 1954, pp.118, 121.

Typical Applications of 4Epoxy' Resins, The Rubber Sand PlasticsAge,February 19'54, pp."8l1i87.V

EpoXiesf-No Wonder, Modern Plastics.` October 1952; pp. 89-94.

